Magnetic element utilizing spin transfer and an MRAM device using the magnetic element

ABSTRACT

A method and system for providing a magnetic element capable of being written using spin-transfer effect while generating a high output signal and a magnetic memory using the magnetic element are disclosed. The magnetic element includes a first ferromagnetic pinned layer, a nonmagnetic spacer layer, a ferromagnetic free layer, an insulating barrier layer and a second ferromagnetic pinned layer. The pinned layer has a magnetization pinned in a first direction. The nonmagnetic spacer layer is conductive and is between the first pinned layer and the free layer. The barrier layer resides between the free layer and the second pinned layer and is an insulator having a thickness allowing electron tunneling through the barrier layer. The second pinned layer has a magnetization pinned in a second direction. The magnetic element is configured to allow the magnetization of the free layer to change direction due to spin transfer when a write current is passed through the magnetic element.

FIELD OF THE INVENTION

The present invention relates to magnetic memory systems, and moreparticularly to a method and system for providing an element thatemploys a spin transfer effect in switching and that can be used in amagnetic memory such as magnetic random access memory (“MRAM”).

BACKGROUND OF THE INVENTION

Magnetic memories are often used in storing data. One type of memoryelement currently of interest utilizes magnetoresistance of a magneticelement for storing data. FIGS. 1A and 1B depict conventional magneticelements 1 and 1′. The conventional magnetic element 1 is a spin valve 1and includes a conventional antiferromagnetic layer 2, a conventionalpinned layer 4, a conventional spacer layer 6 and a conventional freelayer 8. The conventional pinned layer 4 and the conventional free layer8 are ferromagnetic. The conventional spacer layer 6 is nonmagnetic. Theconventional spacer layer 6 is conductive. The antiferromagnetic layer 2is used to fix, or pin, the magnetization of the pinned layer 4 in aparticular direction. The magnetization of the free layer 8 is free torotate, typically in response to an external field.

The conventional magnetic element 1′ is a spin tunneling junction.Portions of the conventional spin tunneling junction 1′ are analogous tothe conventional spin valve 1. Thus, the conventional magnetic element1′ includes an antiferromagnetic layer 2′, a conventional pinned layer4′, an insulating barrier layer 6′ and a free layer 8′. The conventionalbarrier layer 6′ is thin enough for electrons to tunnel through in aconventional spin tunneling junction 1′.

Depending upon the orientations of the magnetizations of the free layer8 or 8′ and the pinned layer 4 or 4′, respectively, the resistance ofthe conventional magnetic element 1 or 1′, respectively, changes. Whenthe magnetizations of the free layer 8 and pinned layer 4 are parallel,the resistance of the conventional spin valve 1 is low. When themagnetizations of the free layer 8 and the pinned layer 4 areantiparallel, the resistance of the conventional spin valve 1 is high.Similarly, when the magnetizations of the free layer 8′ and pinned layer4′ are parallel, the resistance of the conventional spin tunnelingjunction 1′ is low. When the magnetizations of the free layer 8′ andpinned layer 4′ are antiparallel, the resistance of the conventionalspin tunneling junction 1′ is high.

In order to sense the resistance of the conventional magnetic element1/1′, current is driven through the conventional magnetic element 1/1′.Current can be driven through the conventional magnetic element 1 in oneof two configurations, current in plane (“CIP”) and currentperpendicular to the plane (“CPP”). However, for the conventional spintunneling junction 1′, current is driven in the CPP configuration. Inthe CIP configuration, current is driven parallel to the layers of theconventional spin valve 1. Thus, in the CIP configuration, current isdriven from left to right or right to left as seen in FIG. 1A. In theCPP configuration, current is driven perpendicular to the layers ofconventional magnetic element 1/1′. Thus, in the CPP configuration,current is driven up or down as seen in FIG. 1A or 1B. The CPPconfiguration is used in MRAM having a conventional spin tunnelingjunction 1′ in a memory cell.

FIG. 2 depicts a conventional memory array 10 using conventional memorycells 20. Each conventional memory cell 20 includes a conventionalmagnetic element 1/1′, depicted as a resistor in FIG. 2. Theconventional memory array 10 typically uses a spin tunneling junction1′. The conventional array 10 is shown as including four conventionalmemory cells 20. Each memory cell 20 includes a conventional spintunneling junction 1′ and a transistor 22. The memory cells 20 arecoupled to reading/writing column selection 30 via bit lines 32 and 34and to row selection 50 via word lines 52 and 54. Also depicted arewrite lines 60 and 62 which carry currents that generate externalmagnetic fields for the corresponding conventional memory cells 20during writing. The reading/writing column selection 30 is coupled towrite current source 42 and read current source 40 which are coupled toa voltage supply Vdd 48 via line 46.

In order to write to the conventional memory array 10, the write currentIw 42 is applied to the bit line 32 or 34 selected by thereading/writing column selection 30. The read current Ir 40 is notapplied. Both word lines 52 and 54 are disabled. The transistors 22 inall memory cells are disabled. In addition, one of the write lines 60and 62 selected carries a current used to write to the selectedconventional memory cell 20. The combination of the current in the writeline 60 or 62 and the current in the bit line 32 or 34 generates amagnetic field large enough to switch the direction of magnetization ofthe free layer 8′ and thus write to the desired conventional memory cell20. Depending upon the data written to the conventional memory cell 20,the conventional magnetic tunneling junction 1′ will have a highresistance or a low resistance.

When reading from a conventional cell 20 in the conventional memoryarray 10, the read current Ir 40 is applied instead. The memory cell 20selected to be read is determined by the row selection 50 and columnselection 30. The output voltage is read at the output line 44.

Although the conventional magnetic memory 10 using the conventional spintunneling junction 1′ can function, one of ordinary skill in the artwill readily recognize that there are barriers to the use of theconventional magnetic element 1′ and the conventional magnetic memory 10at higher memory cell densities. In particular, the conventional memoryarray 10 is written using an external magnetic field generated bycurrents driven through the bit line 32 or 34 and the write line 60 or62. In other words, the magnetization of the free layer 8′ is switchedby the external magnetic field generated by current driven through thebit line 32 or 34 and the write line 60 or 62. The magnetic fieldrequired to switch the magnetization of the free layer 8′, known as theswitching field, is inversely proportional to the width of theconventional magnetic element 1′. As a result, the switching fieldincreases for conventional memories having smaller magnetic elements 1′.Because the switching field is higher, the current required to be driventhrough the bit line 32 or 34 and particularly through the write line 60or 62 increases dramatically for higher magnetic memory cell density.This large current can cause a host of problems in the conventionalmagnetic memory 10. For example, cross talk and power consumption wouldincrease. In addition, the driving circuits required to drive thecurrent that generates the switching field at the desired memory cell 20would also increase in area and complexity. Furthermore, theconventional write currents have to be large enough to switch a magneticmemory cell but not so large that the neighboring cells areinadvertently switched. This upper limit on the write current amplitudecan lead to reliability issues because the cells that are harder toswitch than others (due to fabrication and material nonuniformity) willfail to write consistently.

Accordingly, what is needed is a system and method for providing amagnetic memory element which can be used in a memory array of highdensity, low power consumption, low cross talk, and high reliability,while providing sufficient read signal. The present invention addressesthe need for such a magnetic memory element.

SUMMARY OF THE INVENTION

The present invention provides a magnetic element and a magnetic memoryusing the magnetic element. The magnetic element includes a first pinnedlayer, a nonmagnetic spacer layer, a free layer, a barrier layer and asecond pinned layer. The pinned layer has a first magnetization pinnedin a first direction. The nonmagnetic spacer layer is conductive and isbetween the first pinned layer and the free layer. The free layer has asecond magnetization. The barrier layer resides between the free layerand the second pinned layer and is an insulator having a thickness thatallows electrons to tunnel through the barrier layer. The second pinnedlayer has a third magnetization pinned in a second direction. Themagnetic element is configured to allow the second magnetization of thefree layer to change direction due to spin transfer when a write currentis passed through the magnetic element.

According to the system and method disclosed herein, the presentinvention provides a magnetic element and a magnetic memory capable ofbeing written using a more efficient and localized phenomenon whilegenerating a high output signal.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A is a diagram of a conventional magnetic element, a spin valve.

FIG. 1B is a diagram of a conventional magnetic element, a spintunneling junction, such as an element used in a magnetic memory.

FIG. 2 is a diagram of a conventional magnetic memory array.

FIG. 3 is a diagram depicting one embodiment of a magnetic element inaccordance with the present invention.

FIG. 4 is a diagram depicting another, preferred embodiment of amagnetic element in accordance with the present invention.

FIG. 5 is a diagram depicting one embodiment of a magnetic memory inaccordance with the present invention using the magnetic element inaccordance with the present invention.

FIG. 6 is a high-level flow chart depicting one embodiment of a methodin accordance with the present invention for providing a magneticelement in accordance with the present invention.

DETAILED DESCRIPTION OF THE INVENTION

The present invention relates to an improvement in magnetic elements andmagnetic memories, such as MRAM. The following description is presentedto enable one of ordinary skill in the art to make and use the inventionand is provided in the context of a patent application and itsrequirements. Various modifications to the preferred embodiment will bereadily apparent to those skilled in the art and the generic principlesherein may be applied to other embodiments. Thus, the present inventionis not intended to be limited to the embodiment shown, but is to beaccorded the widest scope consistent with the principles and featuresdescribed herein.

As described above, one of the challenges faced in increasing thedensity of conventional magnetic memories is the large current requiredto write to the conventional magnetic memories, such as the conventionalmagnetic memory 10 depicted in FIG. 2 and using the conventionalmagnetic elements 1′ of FIG. 1B. In other words, the current required togenerate the magnetic field that switches the direction of themagnetization of the free layer is large. This large current can beproblematic because it can result in cross talk and high powerconsumption.

In order to overcome some of the issues associated with magneticmemories having a higher density of memory cells, a recently discoveredphenomenon, spin transfer, may be utilized. Current knowledge of spintransfer is described in detail in J. C. Slonczewski, “Current-drivenExcitation of Magnetic Multilayers,” Journal of Magnetism and MagneticMaterials, vol. 159, p. L1-L5 (1996); L. Berger, “Emission of Spin Wavesby a Magnetic Multilayer Traversed by a Current,” Phys. Rev. B, Vol. 54,p. 9353 (1996), and in F. J. Albert, J. A. Katine and R. A. Buhman,“Spin-polarized Current Switching of a Co Thin Film Nanomagnet,” Appl.Phys. Lett., vol. 77, No. 23, p. 3809-3811 (2000). Thus, the followingdescription of the spin transfer phenomenon is based upon currentknowledge in the area and is not intended to limit the scope of theinvention.

The spin-transfer effect arises from the spin-dependent electrontransport properties of ferromagnetic-normal metal multilayers. When aspin-polarized current traverses a magnetic multiplayer in a CPPconfiguration, the spin angular momentum of electrons incident on aferromagnetic layer interacts with magnetic moments of the ferromagneticlayer near the interface between the ferromagnetic and normal-metallayers. Through this interaction, the electrons transfer a portion oftheir angular momentum to the ferromagnetic layer. As a result, aspin-polarized current can switch the magnetization direction of theferromagnetic layer if the current density is sufficiently high(approximately 10⁷-10⁸A/cm²), and if the dimensions of the multilayerare small (approximately less than two hundred nanometers) so that selffield effects are not important. In addition, for spin transfer to beable to switch the magnetization direction of a ferromagnetic layer, theferromagnetic layer must be sufficiently thin, for instance, preferablyless than approximately ten nanometers for Co.

The phenomenon of spin transfer can be used in the CPP configuration asan alternative to or in addition to using an external switching field toswitch the direction of magnetization of the free layer 8 or 8′ of theconventional spin valve 1 or the conventional spin tunneling junction1′, respectively. Spin transfer is a phenomenon which dominates othermechanisms and thus becomes observable when the dimensions of theconventional magnetic element 1/1′ are small, in the range of fewhundred nanometers. Consequently, spin transfer is suitable for higherdensity magnetic memories having smaller magnetic elements 1/1′.

For example, switching the magnetization of the conventional free layer8 in the conventional spin valve 1 using spin transfer is described.Current can be driven from the conventional free layer 8 to theconventional pinned layer 4 to switch the magnetization of theconventional free layer 8 to be parallel to the magnetization of theconventional pinned layer 4. The magnetization of the conventional freelayer 8 is assumed to be initially antiparallel to the conventionalpinned layer 4. When current is driven from the conventional free layer8 to the conventional pinned layer 4, conduction electrons travel fromthe conventional pinned layer 4 to the conventional free layer 8. Themajority electrons traveling from the conventional pinned layer 4 havetheir spins polarized in the same direction as the magnetization of theconventional pinned layer 4. These electrons interact with the magneticmoments of the conventional free layer 8 near the interface between theconventional free layer 8 and the conventional spacer layer 6. As aresult of this interaction, the electrons transfer their spin angularmomentum to the conventional free layer 8. Thus, angular momentumcorresponding to spins antiparallel to the magnetization of theconventional free layer 8 (and parallel to the conventional pinned layer4) is transferred to the conventional free layer. If sufficient angularmomentum is transferred by the electrons, the magnetization of theconventional free layer 8 can be switched to be parallel to themagnetization of the conventional free layer 4.

Alternatively, current can be driven from the conventional pinned layer4 to the conventional free layer 8 to switch the magnetization of theconventional free layer 8 to be antiparallel to the magnetization of theconventional pinned layer 8. In this case the magnetization of the freelayer 8 is assumed to be initially parallel to the pinned layer 4. Whencurrent is driven from the conventional pinned layer 4 to theconventional free layer 8, conduction electrons travel in the oppositedirection. The majority electrons have their spins polarized in thedirection of magnetization of the conventional free layer 8, which isoriginally magnetized in the same direction as the conventional pinnedlayer 4. These majority electrons are transmitted through theconventional pinned layer 4. However, the minority electrons, which havespins polarized antiparallel to the magnetization of the conventionalfree layer 8 and the conventional pinned layer 4, will be reflected fromthe conventional pinned layer 4 and travel back to the conventional freelayer 8. The minority electrons reflected by the conventional pinnedlayer 4 interact with magnetic moments of the conventional free layer 8and transfer at least a portion of their spin angular momentum to theconventional free layer 8. If sufficient angular momentum is transferredby the electrons to the conventional free layer 8, the magnetization ofthe free layer 8 can be switched to be antiparallel to the magnetizationof the conventional pinned layer 4.

Using a current driven through the conventional magnetic elements 1 or1′ in the CPP configuration, spin transfer can switch the direction ofmagnetization of the free layer 8 or 8′, respectively. Thus, spintransfer can be used to write to magnetic elements 1 or 1′ in a magneticmemory by using a current through the conventional magnetic element 1 or1′. The mechanism of spin-transfer writing is, therefore, more localizedand generates less cross talk. Spin transfer is also more reliablebecause spin transfer results in a high effective field in theconventional magnetic elements 1/1′ in a device such as MRAM. Inaddition, for a to magnetic element 1 or 1′ having a small enough size,the current required to switch the magnetization can be significantlyless than the current required to generate a switching field in theconventional magnetic memory 10. Thus, there is less power consumptionin writing.

Although the phenomenon of spin transfer can be used to switch thedirection of the magnetization of the conventional free layer 8/8′, oneof ordinary skill in the art will readily recognize that there areadditional barriers to using the conventional magnetic element 1/1′ in amemory. For the conventional spin valve 1, the CPP configuration resultsin a significantly reduced signal. For example, the magnetoresistanceratio for the CPP configuration of the conventional spin valve 1 is onlyapproximately two percent. In addition, the total resistance of theconventional spin valve 1 is low. Thus, the read signal output by theconventional spin valve 1 is very low. Although spin transfer can beused to write to a conventional spin valve 1, the output signal whenreading from the conventional spin valve 1 is low enough to make itdifficult to use the conventional spin valve 1 in a magnetic memory thatis written using spin transfer.

On the other hand, a conventional spin tunneling junction 1′ typicallyhas a large resistance-area product, with Ra˜kΩμm². A high currentdensity is required to induce the spin-transfer effect could destroythin insulating barrier due to ohmic dissipation. Moreover, the spintransfer has not been observed in the conventional spin tunnelingjunction 1′ at room temperature. The conventional spin tunnelingjunction 1′ having high Ra values may, therefore, not be able to be usedin MRAM using spin transfer to write to the magnetic memory cells.Consequently, one of ordinary skill in the art would recognize that areliable, localized mechanism for writing to magnetic memories havinghigher densities and smaller magnetic elements is still desired.

The present invention provides a magnetic element and a magnetic memoryusing the magnetic element. The magnetic element includes a first pinnedlayer, a nonmagnetic spacer layer, a free layer, a barrier layer and asecond pinned layer. The first and second pinned layers as well as thefree layer are ferromagnetic. The pinned layer has a first magnetizationpinned in a first direction. The nonmagnetic spacer layer is conductiveand is between the first pinned layer and the free layer. The free layerhas a second magnetization. The barrier layer resides between the freelayer and the second pinned layer and is an insulator having a thicknessthat allows tunneling through the barrier layer. The second pinned layerhas a third magnetization pinned in a second direction. The magneticelement is configured to allow the second magnetization of the freelayer to change direction due to spin transfer when a write current ispassed through the magnetic element.

The present invention will be described in terms of a particularmagnetic memory and a particular magnetic element having certaincomponents. However, one of ordinary skill in the art will readilyrecognize that this method and system will operate effectively for othermagnetic memory elements having different and/or additional componentsand other magnetic memories having different and/or other features notinconsistent with the present invention. The present invention is alsodescribed in the context of current understanding of the spin transferphenomenon. Consequently, one of ordinary skill in the art will readilyrecognize that theoretical explanations of the behavior of the methodand system are made based upon this current understanding of spintransfer. One of ordinary skill in the art will also readily recognizethat the method and system are described in the context of a structurehaving a particular relationship to the substrate. However, one ofordinary skill in the art will readily recognize that the method andsystem are consistent with other structures. For example, the presentinvention is described in terms of a bottom spin valve (having a pinnedlayer at the bottom of the spin valve) combined with a top spintunneling junction (having a pinned layer at the top of the spintunneling junction). The present invention is also consistent with a topspin valve and a bottom spin tunneling junction. In addition, the methodand system are described in the context of certain layers beingsynthetic. However, one of ordinary skill in the art will readilyrecognize that other and/or additional layers could be synthetic.

To more particularly illustrate the method and system in accordance withthe present invention, refer now to FIG. 3, which depicts one embodimentof a magnetic element 100 in accordance with the present invention. Notethat other layers, such as seed or capping layers, are not depicted forclarity. The magnetic element 100 is formed on a substrate 101. Themagnetic element 100 includes a first antiferromagnetic layer 102, afirst pinned layer 104, a conductive spacer layer 106, a free layer 108,an insulating barrier layer 110, a second pinned layer 112 and a secondantiferromagnetic layer 114. Note that the first pinned layer 104, thefree layer 108 and the second pinned layer 112 are depicted as singleconstituent ferromagnetic layers. However, one of ordinary skill in theart will readily recognize that any portion of the layers 104, 108 and112 can be synthetic. The magnetic element 100 can be considered to be acombination of a spin valve and a spin tunneling junction. The spinvalve would be considered to include the first antiferromagnetic layer102, the first pinned layer 104, the conductive spacer layer 106, andthe free layer 108. The spin tunneling junction would be considered toinclude the free layer 108, the insulating barrier layer 110, the secondpinned layer 112 and the second antiferromagnetic layer 114. In thepreferred embodiment, the spin valve portion of the magnetic element 100writes to the free layer 108 using spin transfer, while the spintunneling portion of the magnetic element 100 is used to read themagnetic element 100.

The magnetic element 100 is configured to allow the magnetization of thefree layer 108 to be switched using spin transfer. Consequently, thedimensions of the magnetic element 100 are small, in the range of fewhundred nanometers. In a preferred embodiment, the dimensions of themagnetic element 100 are less than two hundred nanometers and preferablyapproximately one hundred nanometers. The magnetic element 100preferably has a depth, perpendicular to the plane of the page in FIG.3, of approximately fifty nanometers. The depth is preferably smallerthan the width of the magnetic element 100 so that the magnetic element100 has some shape anisotropy, ensuring that the free layer 108 has apreferred direction. In addition, the thickness of the free layer 108 islow enough so that the spin transfer is strong enough to rotate the freelayer magnetization into alignment with the magnetizations of the pinnedlayers 104 and 112. In a preferred embodiment, the free layer 108 has athickness of less than or equal to 10 nm.

The pinned layers 104 and 112 and free layer 108 are ferromagnetic. Thepinned layers 104 and 112 as well as the free layer 108 preferablyinclude Co, Fe, Ni and their alloys. Also in a preferred embodiment, thethicknesses of the ferromagnetic layer 104, 108 and 112 are selected tobalance the interaction and demagnetization fields of the ferromagneticlayers so that the free layer 108 does not experience a strong net bias.In other words, the total of the magnetostatic field and the interlayerand static coupling fields preferably sum to zero to reduce the bias onthe free layer 108. Moreover, as discussed above, the magnetic elementpreferably has some shape anisotropy so that the free layer 108 has apreferred direction. In addition, a seed layer (not shown), such as Taor NiFeCr is preferably provided under the antiferromagnetic layer 10 toensure that the antiferromagnetic layer 102 has the desired structureand properties. The conductive spacer layer 106 is preferably Cu orother nonmagnetic transition metal. The barrier layer 110 is thin enoughto allow the tunneling of electrons through the barrier layer 110 and ispreferably composed of alumina. In alternate embodiments, the barrierlayer 110 may include other dielectric materials including, but notlimited to, AlN, Ta₂O₅, SiO₂, HfO₂, ZrO₂, MgO, MgF₂ and CaF₂.

The magnetizations of the first pinned layer 104 and the second pinnedlayer 112 are depicted as being pinned in opposite directions. In analternate embodiment, the magnetizations of the pinned layers 104 and112 may be pinned in the same direction. For example, if a syntheticfree layer is used, the pinned layers 104 and 112 are preferably pinnedin the same direction. However, in an embodiment which may not functionas well as the preferred embodiment, the pinned layers 104 and 112 maystill be pinned in the same direction when a simple free layer is used.Thus, in a preferred embodiment, the pinned layers 104 and 112 adjacentto the spacer layer 106 and the barrier layer 110, respectively, aredesired to be aligned in opposite directions. This orientation isdesired so that if the spin tunneling junction portion of the magneticelement 100 can be made to contribute to spin transfer, as describedbelow, then the magnetic element can be written as desired by using asmaller current density.

The antiferromagnetic layers 102 and 114 are used to pin themagnetizations of the pinned layers 104 and 112, respectively. Theantiferromagnetic layers 102 and 114 are preferably composed of PtMn.However, nothing prevents the antiferromagnetic layers 102 and 114 fromincluding other antiferromagnetic materials, such as NiMn, PdMn andIrMn. PtMn is preferred for use in the anti ferromagnetic layers 102 and114 because PtMn has a high blocking temperature and a high exchangebiasing field, which improve the thermal stability of the magneticelement 100. In such an embodiment, the orientation of theantiferromagnetic layers can be set by annealing the magnetic element100 in a field of at least five thousand Oersted at approximately twohundred and seventy degrees Celsius for between three and ten hours.Antiferromagnetic layers 102 and 114 having different blockingtemperatures are preferably used when the pinned layers 104 and 112 areto be pinned in different directions. When the antiferromagnetic layer102 has a higher blocking temperature than the anti ferromagnetic layer114, the orientation of the antiferromagnetic layer 114 can be setindependently from the antiferromagnetic layer 102 by annealing themagnetic element 100 at the blocking temperature of theantiferromagnetic layer 114. As a result, the magnetizations of thepinned layers 102 and 114 can be pinned in different directions.

In operation, the magnetic element 100 is written by using spintransfer. Currently, the spin transfer phenomenon is predominantlyprovided using the spin valve portion of the magnetic element 100. Inparticular, a current can be driven from the second pinned layer 112through the free layer 108 and the first pinned layer 104. Such acurrent corresponds to electrons spin polarized in the direction ofmagnetization of the first pinned layer 104 and can thus set themagnetization of the free layer 108 in the same direction as the firstpinned layer 104. Similarly, when is current driven in the oppositedirection, minority electrons reflecting off of the first pinned layer104 and returning to the free layer 108 can switch the magnetization ofthe free layer 108 to be opposite to the magnetization of the pinnedlayer 104.

Consequently, spin transfer can be used to write to magnetic element100. As a result, a switching field driven by an external current isunnecessary. Instead, a more localized and reliable phenomenon is usedto write to the magnetic element 100. In addition, for a magneticelement 100 having the preferred dimensions, a sufficient currentdensity on the order of 10⁷ Amps/cm² can be provided at a relativelysmall current. For example, a current density of approximately 10⁷Amps/cm² can be provided with a current of approximately 0.5 mA for amagnetic element having an ellipsoidal shape of 0.06×0.12 μm ². As aresult, the use special circuitry for delivering very high currents maybe avoided.

Additional advances in spin tunneling junctions with low Ra of few Ωμm²may allow the spin tunneling junction portion (108, 110, 112 and 114) ofthe magnetic element 100 to contribute to the spin transfer because theelectron spin is conserved during tunneling. Consequently, in apreferred embodiment, the magnetizations of the pinned layers 104 and112 are in opposite directions so that the spin tunneling junctionportion of the magnetic element 100 has an opportunity to appropriatelycontribute to the spin transfer. In such an embodiment, the spintransfer due to conduction electrons traveling from the pinned layer 104to the free layer 108 and conduction electrons reflected off the pinnedlayer 112 and returning to the free layer 108 would work together toalign the magnetization of the free layer 108 in the direction ofmagnetization of the pinned layer 104. Similarly, spin transfer due toconduction electrons traveling from the pinned layer 112 to the freelayer 108 and conduction electrons reflected off the pinned layer 104and returning to the free layer 108 would work together to align themagnetization of the free layer 108 in the direction of magnetization ofthe pinned layer 112. Because the magnetizations of the pinned layers104 and 112 are pinned in opposite directions, in such an embodimentusing advances in spin tunneling junctions would improve the ability ofthe magnetic element 100 to be written using spin transfer. In such anembodiment, the current required to switch the direction ofmagnetization of the free layer 108 may be further reduced, for exampleby a factor of two.

During reading, the properties of the spin tunneling junction portion ofthe magnetic element 100 are preferably exploited. Because of theexistence of the insulating barrier 110 and the second pinned layer 112,the spin tunneling portion of the magnetic element 100 dominates theoutput signal. In other words, although writing to the magnetic element100 sets the magnetization of the free layer 108 with respect to thefirst pinned layer 104, the magnetization of the free layer 108 withrespect to the second pinned layer 110 dominates the output signal ofthe magnetic element, both in total resistance and in magnetoresistancechanges. Thus, during reading, the state of the free layer 108 withrespect to the second pinned layer 112 (antiparallel or parallel to thesecond pinned layer 112) determines the output of the magnetic element100. When the free layer 108 is parallel to the second pinned layer 112,the resistance of the magnetic element 100 is low. When the free layer108 is antiparallel to the second pinned layer 112, the resistance ofthe magnetic element 100 is high. Ra for the magnetic element 100 ispreferably on the order of few Ωμm² As a result, a higher currentdensity, on the order of 10Amps/cm², can be provided without destroyingthe magnetic element 100. Moreover, because the magnetoresistance due tothe spin tunneling junction portion of the magnetic element 100 is muchlarger than (preferably at least twenty times) that from spin-valveportion in the CPP configuration, the magnetic element 100 provides asufficient signal at lower current densities in the CPP configuration.

The magnetic element 100 can thus be written to and read from using acurrent driven through the magnetic element 100. The read current driventhrough the magnetic element 100 is less than the current driven throughthe magnetic element 100 during writing. The read current is less thanthe write current in order to ensure that the direction of magnetizationof the free layer 108 magnetic element 100 is not inadvertently switchedto during reading. In a preferred embodiment, the read current is anorder of magnitude less than the write current.

Thus, the magnetic element 100 can be written by exploiting thespin-transfer phenomenon. Because spin transfer is used, an externalcurrent producing an external switching magnetic field is no longerneeded to write to the free layer 108 of the magnetic element 100.Instead, a current driven through the magnetic element 100 is used. As aresult, there is less cross talk because a more localized switchingmechanism is utilized, and less power consumed. In addition, spintransfer has been found to be a more reliable switching mechanism thanan external switching field. Spin transfer generates a very higheffective field and can thus switch a greater percentage of magneticelements 100 in a memory. Furthermore, for a magnetic element having thepreferred size, the current required to write to the magnetic element100 may be reduced. The magnetic element 100 also has a significantlyhigher output signal when being read in a CPP configuration than aconventional spin valve because of the presence of the spin tunnelingjunction portions (layers 108, 110, 112 and 114) of the magnetic element100. Consequently, the magnetic element 100 is suitable for use as astorage element in a higher density magnetic memory such as MRAM.

FIG. 4 is a diagram depicting another, preferred embodiment of amagnetic element 100′ in accordance with the present invention. Themagnetic element 100′ has many of the same components as the magneticelement 100 depicted in FIG. 3. Consequently, analogous structures arelabeled similarly for the magnetic element 100′ depicted in FIG. 4. Inaddition, these components are preferably fabricated in an analogousmanner and made from similar materials as analogous components in themagnetic element 100. However, the second pinned layer 112′ of themagnetic element 100′ is a synthetic pinned layer 112′. Thus, thesynthetic pinned layer 112′ includes ferromagnetic layers 111 and 115separated by a nonmagnetic conductive spacer layer 113. The magneticlayers 111 and 115 preferably include Co, Fe, Ni and their ferromagneticalloys, such as NiFe, CoFe or CoNiFe. In addition, the above materialscontaining some B impurities may also be used for the magnetic layers111 and 115. The B impurities give the materials greater thermalstability. In alternate embodiments, the layers 111 and 115 may be madeof other magnetic materials such as half metallic ferromagnets includingCrO₂, NiMnSb and PtMnSb. The nonmagnetic spacer layer 113 preferablyincludes materials such as Ru, Ir and Re. The thickness of thenonmagnetic spacer layer is such that the ferromagnetic layers 111 and115 are antiferromagnetically coupled.

The synthetic pinned layer 112′ is preferred to simplify the annealingprocess which sets the pinning directions of the magnetizations of thesecond pinned layer 112′ and the first pinned layer 104′. In particular,use of the synthetic pinned layer 112′ allows the antiferromagneticlayers 102′ and 114′ to be made from the same material, preferably PtMn,and aligned in the same direction. The antiferromagnetic layers 102′ and114′ may thus be aligned together in the same step. Consequently, themagnetizations of the first pinned layer 104′ and the ferromagneticlayer 115 are pinned in the same direction. The magnetization of theferromagnetic layer 111 is in the opposite direction as themagnetization of the ferromagnetic layer 115 and the pinned layer 104′.As a result, the desired directions of the magnetizations of theferromagnetic layers 104′ and 111 adjacent to the spacer layer 106′ andbarrier layer 110′, respectively, are more easily established.

Thus, the magnetic element 100′ can also be written by exploiting thespin-transfer phenomenon. Because spin transfer is used, an externalcurrent producing an external switching magnetic field is no longerneeded to write to the free layer 108′ of the magnetic element 100′. Asa result, the mechanism used in switching the magnetization of the freelayer 108′ is more localized. In addition, spin transfer can switch agreater percentage of the magnetic memory elements and is thus morereliable. Further, for a magnetic element 100′ having the preferredsize, the current required to write to the magnetic element 100′ may begreatly reduced over the current used in writing to the conventionalmagnetic element 1′ in a higher density memory. In addition, setting thepinning directions of the magnetic element 100′ is simplified due to thepresence of the synthetic pinned layer 112′, which allows the use of thesame antiferromagnetic materials such as PtMn for both pinned layers104′ and 112′. The use of PtMn antiferromagnetic materials greatlyimproves the thermal stability of the magnetic element 100′.Furthermore, the magnetic element 100 also has a significantly higheroutput signal when being read in a CPP configuration than a conventionalspin valve because of the presence of the spin tunneling junctionportion (layers 108′, 110′, 112′ and 114′) of the magnetic element 100′.Consequently, the magnetic element 100′ is suitable for use as a storageelement in a higher density magnetic memory such as MRAM.

FIG. 5 is a diagram depicting one embodiment of a magnetic memory array150 in accordance with the present invention using the magnetic element100 or 100′ in accordance with the present invention. The magneticmemory array 150 is for exemplary purposes only and thus depicts amemory array 150 in which the magnetic elements 100 or 100′ may be moredirectly incorporated into a conventional memory. Thus, each memory cell160 includes a magnetic element 100 or 100′ and a transistor 162. Themagnetic memory array 150 also includes row selection mechanism 170,column selection mechanism 180, word lines 172 and 174, and bit lines182 and 184. The magnetic memory array 150 further includes writecurrent source 190 and read current source 192. However, the magneticmemory array 150 does not include any write lines.

Because spin transfer is used to write to the magnetic elements 100 and100′, additional lines, such as write lines 60 and 62 of theconventional memory 10, depicted in FIG. 2, are unnecessary.Consequently, the density of the magnetic memory 150 may be furtherincreased without high power consumption and other issues due to the useof the conventional write operation to write the conventional memoryelements 1 and 1′. In addition, the circuitry used to write to themagnetic elements 100/100′ can be simplified because of the omission ofseparate write lines.

FIG. 6 is a high-level flow chart depicting one embodiment of a method200 in accordance with the present invention for providing a magneticelement in accordance with the present invention. For clarity, themethod 200 is described in the context of the magnetic element 100.However, the method 200 could be adapted to other magnetic elements. Theantiferromagnetic layer 102 is provided, via step 202. In a preferredembodiment, the antiferromagnetic layer 102 is provided on theappropriate seed layer. The first pinned layer 104 and the conductivespacer 106 are provided in steps 204 and 206, respectively. Step 204could also include providing a synthetic pinned layer. The free layer108 and barrier layer 110 are provided in steps 208 and 210,respectively. Step 208 could include providing a synthetic free layer.The second pinned layer 112 and second antiferromagnetic layer 114 areprovided, via steps 212 and 214, respectively. The orientation(s) of theantiferromagnetic layers 114 and 102 are set, via step 216. Step 216could include independently setting the orientations of theantiferromagnetic layers 102 and 114 if the pinned layers 104 and 112are to be pinned in different directions. Similarly, step 216 could setthe orientations of the antiferromagnetic layers 102 and 114 together.The magnetic element 100 may then be defined and other processingcompleted. Thus, using the method 200, the magnetic elements 100 and/or100′ may be fabricated.

A method and system has been disclosed for providing a magnetic elementthat can be written using spin transfer, and thus a smaller andlocalized switching current, and which provides an adequate read signal.Although the present invention has been described in accordance with theembodiments shown, one of ordinary skill in the art will readilyrecognize that there could be variations to the embodiments and thosevariations would be within the spirit and scope of the presentinvention. Accordingly, many modifications may be made by one ofordinary skill in the art without departing from the spirit and scope ofthe appended claims.

What is claimed is:
 1. A magnetic element comprising: a first pinnedlayer, the pinned layer being ferromagnetic and having a firstmagnetization, the first magnetization being pinned in a firstdirection; a nonmagnetic spacer layer, the nonmagnetic spacer layerbeing conductive; a free layer, the first nonmagnetic spacer layerresiding between the first pinned layer and the free layer, the freelayer being ferromagnetic and having a second magnetization; a barrierlayer, the barrier layer being an insulator and having a thickness thatallows tunneling through the barrier layer; a second pinned layer, thesecond pinned layer being ferromagnetic and having a third magnetizationpinned in a second direction, the barrier layer being between the freelayer and the second pinned layer; wherein the magnetic element isconfigured to allow the second magnetization of the free layer to changedirection due to spin transfer when a write current is passed throughthe magnetic element.
 2. The magnetic element of claim 1 wherein thefirst direction is opposite to the second direction.
 3. The magneticelement of claim 1 further comprising: a first antiferromagnetic layeradjacent to the first pinned layer, the first antiferromagnetic layerfor pinning the first magnetization of the first pinned layer; and asecond antiferromagnetic layer adjacent to the second pinned layer, thesecond antiferromagnetic layer for pinning the third magnetization ofthe second pinned layer.
 4. The magnetic element of claim 3 wherein thefirst antiferromagnetic layer and the second antiferromagnetic layerinclude PtMn.
 5. The magnetic element of claim 3 wherein the firstantiferromagnetic layer has a first blocking temperature, the secondantiferromagnetic layer has a second blocking temperature, the secondblocking temperature being different from the first blockingtemperature.
 6. The magnetic element of claim 1 wherein the first pinnedlayer is a synthetic pinned layer.
 7. The magnetic element of claim 1wherein the second pinned layer is a synthetic pinned layer.
 8. Themagnetic element of claim 1 wherein the magnetic element has a width ofless than or equal to 200 nm.
 9. The magnetic element of claim 8 whereinthe magnetic element has a width of approximately 100 nm.
 10. Themagnetic element of claim 9 wherein the magnetic element has a depth ofapproximately 50 nm.
 11. A magnetic memory device comprising: aplurality of magnetic cells including a plurality of magnetic elements,each of the plurality of magnetic elements including a first pinnedlayer, a nonmagnetic spacer layer, a free layer, a barrier layer, and asecond pinned layer, the first pinned layer being ferromagnetic andhaving a first magnetization, the first magnetization being pinned in afirst direction, the nonmagnetic spacer layer being conductive andresiding between the first pinned layer and the free layer, the freelayer being ferromagnetic and having a second magnetization, the barrierlayer being an insulator and having a thickness that allows tunnelingthrough the barrier layer, the second pinned layer being ferromagneticand having a third magnetization pinned in a second direction, thebarrier layer being between the free layer and the second pinned layer,each of the plurality of magnetic elements being configured such thatthe second magnetization of the free layer can change direction due tospin transfer when a write current is passed through the magneticelement; a plurality of row lines coupled to the plurality of magneticcells; and a plurality of column lines coupled with the plurality ofcells, the plurality of row lines and the plurality of column lines forselecting a portion of the plurality of magnetic cells for reading andwriting.
 12. The magnetic memory device of claim 11 wherein the magneticmemory is configured to write to the portion of the plurality ofmagnetic cells without requiring the use of additional lines.
 13. Themagnetic memory device of claim 11 wherein the first direction of thefirst magnetization of the first pinned layer is opposite to the seconddirection of the third magnetization of the second pinned layer.
 14. Themagnetic memory device of claim 11 wherein each of the plurality ofmagnetic elements further include: a first antiferromagnetic layeradjacent to the first pinned layer, the first antiferromagnetic layerfor pinning the first magnetization of the first pinned layer; and asecond antiferromagnetic layer adjacent to the second pinned layer, thesecond antiferromagnetic layer for pinning the third magnetization ofthe second pinned layer.
 15. The magnetic memory device of claim 14wherein the first antiferromagnetic layer and the secondantiferromagnetic layer include PtMn.
 16. The magnetic memory device ofclaim 14 wherein the first antiferromagnetic layer has a first blockingtemperature, the second antiferromagnetic layer has a second blockingtemperature, the second blocking temperature being different from thefirst blocking temperature.
 17. The magnetic memory device of claim 11wherein the first pinned layer is a synthetic pinned layer.
 18. Themagnetic memory device of claim 11 wherein the second pinned layer is asynthetic pinned layer.
 19. The magnetic memory device of claim 11wherein each of the plurality of magnetic elements has a width of lessthan or equal to 200 nm.
 20. The magnetic memory device of claim 19wherein each of the plurality of magnetic elements has a width ofapproximately 100 nm.
 21. The magnetic memory device of claim 20 whereinthe magnetic element has a depth of approximately 50 nm.